Static Fourier transform infrared spectrometer

Abstract

Fourier transform spectroscopy has established itself as the standard method for spectral analysis of infrared light. Here we present a robust and compact novel static Fourier transform spectrometer design without any moving parts. The design is well suited for measurements in the infrared as it works with extended light sources independent of their size. The design is experimentally evaluated in the mid-infrared wavelength region between 7.2 μm and 16 μm. Due to its large etendue, its low internal light loss, and its static design it enables high speed spectral analysis in the mid-infrared.

Gas Measurement Using Static Fourier Transform Infrared Spectrometers

“…Before discussing the gas measurement system in detail, we give a brief explanation of the sSMFTS principle [ 11 ]. Figure 1 shows a schematic overview of the spectrometer, where only the focal rays of a diverging light source are indicated.…”

“…Whereas the non-linear part of the OPD has to be calculated numerically using ray tracing, the linear OPD can be determined via Equation ( 5 ), where s is the separation of the virtual sources according to Figure 1 , f is the focal length of the Fourier lens and y is the position on the detector array in y -direction. The 1D-interferogram can then be computed by a special averaging technique described in [ 11 ]. …”

“…As every optical component in this setup is passed only once, the sSMFTS shows low internal light loss. Additionally, due to the source-doubling principle, light sources of any size can be used without reducing the visibility of the interferogram or restricting spectral resolution, allowing high signal-to-noise ratios [ 11 ]. As the complete interferogram is modulated spatially and thus can be recorded at once, measurement speed is only limited by the detector electronics.…”

“…A more detailed description of the spectrometer is given in [ 11 ]. For the measurement system presented in this paper, we use a zinc selenide beam splitter with a thickness of 3 mm, a germanium Fourier lens with a focal length f of 40 mm and a silicon based microbolometer array with 640 px × 480 px as well as a pixel pitch of 17 m. As we place the central peak of the interferogram at around 20% of the detector length, the maximum linear OPD of the setup amounts to 1.3 mm according to Equation ( 5 ), with the separation s of the virtual sources being 6 mm.…”

“…Thus, for process monitoring in industrial environments, the use of static systems offers certain benefits. Static single-mirror Fourier transform spectrometers (sSMFTS) [ 11 , 12 ] show a rugged and compact design with high SNR, as they work without moving parts and still are based on the Fourier transform principle. Temperature effects can be compensated with suitable algorithmic corrections [ 13 ].…”